DMT alters cortical travelling waves

This EEG study (n=13) finds that DMT elicited similar brain activation (cortical travelling waves) as visual stimulation does. This provides extra proof for the REBUS-model of psychedelics.

Abstract

“Psychedelic drugs are potent modulators of conscious states and therefore powerful tools for investigating their neurobiology. N,N, Dimethyltryptamine (DMT) can rapidly induce an extremely immersive state of consciousness characterized by vivid and elaborate visual imagery. Here, we investigated the electrophysiological correlates of the DMT induced altered state from a pool of participants receiving DMT and (separately) placebo (saline) while instructed to keep their eyes closed. Consistent with our hypotheses, results revealed a spatio-temporal pattern of cortical activation (i.e., travelling waves) similar to that elicited by visual stimulation. Moreover, the typical top-down alpha-band rhythms of closed-eyes rest were significantly decreased, while the bottom-up forward wave was significantly increased. These results support a recent model proposing that psychedelics reduce the ‘precision-weighting of priors’, thus altering the balance of top-down versus bottom-up information passing. The robust hypothesis-confirming nature of these findings imply the discovery of an important mechanistic principle underpinning psychedelic-induced altered states.”

Authors: Andrea Alamia, Christopher Timmermann, Rufin VanRullen & Robin L. Carhart-Harris

Notes

This study builds further on earlier brain-imaging (EEG) studies and uses the same data (participants) as Timmermann et al (2019) which studied the brain under DMT influence. The current study particularly looks at ‘travelling wave’ (forward, top-down) “fronts of rhythmic activity which propagate across regions in the cortical visual hierarchy.“

This paper fits with/finds more support for the ‘Relaxed Beliefs Under Psychedelics’ (REBUS) model proposed by co-author of this paper Carhart-Harris and Friston (2019).

“These are the first EEG data on the effects of DMT on human resting state brain activity. In line with prior hypothesis, clear evidence was found of a shift in cortical travelling waves away from the normal basal predominance of backward waves and towards the predominance of forward waves – remarkably similar to what has been observed during eyes-open visual stimulation. Moreover, the increases in forward waves correlated positively with both the general intensity of DMT’s subjective effects, as well as its more specific effects on eyes-closed visual imagery. These findings have specific and broad implications: for the brain mechanisms underlying the DMT/psychedelic state as well as conscious visual perception more broadly.”

Summary

Abstract

Psychedelic drugs can induce an extremely immersive state of consciousness characterized by vivid and elaborate visual imagery. In this study, we investigated the electrophysiological correlates of the DMT-induced altered state, and found that the typical top-down alpha-band rhythms of closed-eyes rest were significantly decreased, while the bottom-up ‘forward travelling wave’ was significantly increased.

Introduction

DMT is a classic psychedelic drug that is taken exogenously by humans to alter the quality of their consciousness. It is metabolized in the gastrointestinal (GI) system before reaching the brain.

Previous studies of ayahuasca-induced brain changes have reported broad decreases in oscillatory power, while the most marked decreases occur in the -band oscillations (8-12 Hz). DMT-induced brain changes have been shown to cause increases in signal diversity and decreased alpha power.

Normal brain function reveals distinct spatio-temporal dynamics during visual perception, which differ considerably from those observed during closed-eyes restfulness. These dynamics can be described as oscillatory ‘travelling waves’.

DMT’s visual effects lend themselves particularly well to testing a hypothesis that psychedelics affect travelling waves by decreasing top-down processing and increasing bottom-up signal passing.

Here we quantified the amount and direction of travelling waves in healthy participants who received DMT intravenously, during eyes-closed conditions, and hypothesized that this effect correlates with the vivid ‘visionary’ component of the DMT experience.

Results

We measure the waves’ amount and direction with a method devised in our previous studies16,17. We generate 2D maps from the EEG signals by stacking the signals from 5 central mid-line electrodes and compute a 2D-FFT, from which we extract the maximum values, representing the raw amount of FW and BW waves.

The net amount of waves against the null distribution is the most informative value, but a direct comparison between FW and BW waves is not readily interpretable.

DMT alters the cortical pattern of travelling waves, and we collected EEG recordings 5 minutes prior to drugs administration and up to 20 minutes after. During quiet closed-eyes restfulness, BW waves spread from higher to lower regions, whereas no significant waves propagate in the opposite FW direction. However, after DMT injection, the cortical pattern changed drastically: BW waves decreased but remained significantly above zero, whereas FW waves increased significantly above zero. In line with previous studies, we observed an increase in FW waves and a decrease in BW waves after intravenous DMT injection. Although a direct comparison is not statistically possible, we found similarities between the dynamics elicited by DMT injection and those observed in another study17.

We recently showed that FW waves increase during visual stimulation, whereas BW waves decrease, in line with their putative functional role in information transmission. The results of this study show that DMT produces spatio-temporal dynamics similar to those elicited by true visual stimulation.

We investigated whether DMT influences not only the waves’ direction but also their frequency spectrum. We found that DMT significantly changed the waves’ spectrum, with a significant reduction in the alpha-band, coupled with an increase in the delta and theta bands.

After DMT injection, FW and BW waves are present in the brain in equal amounts, but the ratio of contribution from each changes. This result demonstrates that FW waves are weaker whenever BW waves are stronger, and vice versa.

We investigated whether changes in travelling waves under DMT correlated with the subjective effects of the drug. We found that intensity ratings and wave amplitudes correlated positively (FW) or negatively (BW) across time, both peaking a few minutes after drug injection.

Subjects who reported the most intense effects also had the strongest travelling waves in the FW direction, and the weakest waves in the BW direction. These results strongly support the view that higher amounts of FW waves correlate with conscious visual experiences.

Discussion

In this study, participants who received DMT experienced a decrease in band 8-12Hz oscillatory frequencies, an increase in FW waves, and a reduction in BW waves. These changes were correlated with subjective intensity of the drug experience and visual imagery.

Western medicine has explored the therapeutic potential of psychedelic compounds for over a century, but there has been a surprising dearth of resting-state human neuroimaging studies of pure DMT.

Previous work has shown that ayahuasca increases visual cortex BOLD signal while participants engage in an eyes-closed imagery task, and that increased FW travelling waves correlate with visionary experiences. The present findings support the notion that DMT engages the visual apparatus in a fashion that is consistent with actual exogenously driven visual perception.

DMT’s signature psychological effects are likely mediated by stimulation of the serotonin 2A (5-HT2A) receptor subtype, which has also been found to be essential for the full signature psychological and brain effects of Ayahuasca 10.

Predictive coding is a mechanism by which the brain strives to be a model of its environment. Psychedelic compounds alter this mechanism by decreasing the precision weighting of top-down priors, thereby liberating bottom-up information flow.

The discovery of cortical travelling waves provided support for the assumption that top-down predictions and bottom-up prediction-errors are encoded in the direction of propagation of cortical travelling waves.

These findings indicate that the brain’s normal basal predominance of backward waves shifts towards the predominance of forward waves during the DMT/psychedelic state and during eyes-closed visual imagery.

Methods

Thirteen participants took part in this study, which analyzed a dataset presented in a previous publication. It was approved by the National Research Ethics Committee London – Brent and the Health Research Authority.

Participants were carefully screened before joining the experiments, and underwent physical examination, electrocardiogram, blood pressure and routine blood tests. A successful psychiatric interview was necessary to join the experiment, and the day before the experiment a urine and pregnancy test was performed, together with a breathalyzer test.

EEG signals were recorded using a 32-channels Brainproduct EEG system sampling at 1000Hz. Artifacts were removed using a high-pass filter and an anti-aliasing low-pass filter, and ICA was performed to remove components corresponding to eye-movement and cardiac-related artifacts.

We epoch the preprocessed EEG signals in 1 second windows, sliding with a step of 500ms, and then arranged a 2D time-electrode map composed of 5 electrodes. We computed the 2D Fast Fourier Transform, and extracted the maximum value in the upper and lower quadrants, representing respectively the power of forward (FW) and backward (BW) waves.

We performed statistical analysis on the EEG signals of FW and BW conditions, and corrected all the p-values according to the False Discovery Rate.